Process for reducing the interdiffusion of conductors and/or semiconductors in contact with each other

ABSTRACT

The interdiffusion at the interface between a metallic-type conductor or semiconductor and a second metallic-type conductor or semiconductor in intimate contact therewith due to elevated temperatures is reduced by subjecting a surface of at least one of the materials when at an elevated temperature to a gas which contains at least one component which changes the surface potential and work function of the surface which is subjected to the gas in a direction according to the relative electronegativities of the materials.

DESCRIPTION

1. Technical Field

The present invention is concerned with reducing the interdiffusion whensubjected to elevated temperatures of a metallic-type electricalconductor or semiconductor and a second and different metallic-typeelectrical conductor or semiconductor which are in intimate contact witheach other. The process of the present invention is suitable for variousprocessing steps or preparing an article which involves elevatedtemperatures and is also suitable for preventing interdiffusion duringthe operation of an article at elevated temperatures.

The process of the present invention is especially advantageous for useduring the fabrication of metallic contacts to metallic-type conductorsand/or semiconductors, and particularly when shallow junctions orinterfaces are involved. The present invention is particularly concernedwith employing certain controlled gaseous atmospheres during the timethe article being treated is subjected to elevated temperatures.

2. Background Art

Typical semiconductor devices are multilayer structures which include asubstrate of a semiconductor upon which is provided an intermediatelayer of a conductor or semiconductor on top of which is providedanother conductor or semiconductor layer of a material different fromthat of the intermediate layer. In addition, electrical contacts arenormally provided on at least the top surface of the upper layer. Also,electrical contact might be provided on the underside of thesemiconductor substrate. These electrical contacts are usually providedby subjecting the multilayer structure and material for the contacts toelevated temperatures to cause fusion of the materials.

This type of procedure, however, is conducive to interdiffusion ormigration of the interface of the material of the intermediate layersand the material of the top layer. This interdiffusion, in turn, canresult in material from the intermediate layer contaminating the topsurface of the upper layer thereby significantly altering the electricalcharacteristics of the article. This can also result in the materialfrom the top layer migrating all the way to the underside of thesemiconductor substrate, thereby causing short circuiting.

In order to minimize the interdiffusion, the heating is normally carriedout in an inert atmosphere. However, the problem of interdiffusion stillpersists and is especially detrimental when preparing devices havingshallow junctions or interfaces.

In addition, when various semiconductor devices are operated, dependingupon their particular intended use, they may be subjected to very hightemperatures. Thus, the operation, when at elevated temperature anddepending upon the environment, can eventually result in failure of thedevice due to interdiffusion of adjacent conductive and/orsemiconductive layers.

DISCLOSURE OF INVENTION

The present invention is concerned with reducing the interdiffusion whensubjected to elevated temperatures of a first metallic-type conductor orsemiconductor and a second and different metallic-type electricalconductor or semiconductor which is adjacent to and in intimate contactwith the first metallic-type electrical conductor and/or semiconductor.The first metallic-type electrical conductor and/or semiconductor andthe second and different metallic-type electrical conductor and/orsemiconductor have different electronegativities. The electronegativityof a material is the measure of the ability of the material to attractelectrons. The greater the ability, the higher the electronegativity ofthe material.

The term "metallic-type" as used herein refers to electronicallyconductive metals, electrically conductive mixtures of metals,electrically conductive metal alloys, as well as nonmetallic materials,such as highly doped polycrystalline silicon or intermetallic silicideswhich, nevertheless, have electrical conductivities of the magnitudegenerally possessed by metals.

The process of the present invention comprises subjecting or exposing atleast one surface of at least one of the metallic-type electricalconductors or semiconductors when at elevated temperature to a gas. Thegas contains at least one type of molecule which changes the surfacepotential of the exposed metallic-type conductor and/or semiconductor byincreasing its work function when it is more electronegative than theother metallic-type conductor and semiconductor or by decreasing itswork function when it is less electro-negative than the othermetallic-type conductor or semiconductor, thereby reducing theinterdiffusion of the conductors and/or semiconductors.

The work function is the theshold energy needed to release the electronfrom the surface. Work function can be measured by the energy of lightneeded to cause release of an electron from the surface of a particularmaterial according to procedure specified in B. Gysae and S. Wagener; Z.Physik, 115,296 (1940); and J. C. P. Mignolet: Discussions FaradaySociety, 8, 326 (1950).

DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are graphs illustrating the backscattering resultsobtained by subjecting Au--Cr--Al₂ O₃ structures to elevatedtemperatures in different atmospheres.

BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION

The structures which are subjected to the process of the presentinvention include at least two layers adjacent to and in intimatecontact with each other. One of the layers contains a firstmetallic-type electrical conductive material and/or semiconductor, whilethe other layer contains a second and different metallic-type electricalconductive material and/or semiconductor. The electronegativity of thefirst conductor and/or semiconductor is different from that of thesecond conductor and/or semiconductor.

The metallic-type material can be an electrically conductive metal,mixture of electrically conductive metals, electrically conductivemetallic alloys, as well as nonmetallic materials, such as highly dopedpolysilicon or intermetalic silicides which, nevertheless, haveelectrical conductivities of the magnitude generally possessed bymetals. In addition, the metallic-type layer can contain nonconductingmaterials as long as such do not destroy the conductivitycharacteristics of the layer to an undesired extent.

Examples of some metallic-type materials advantageously processedaccording to the present invention include gold, chromium, cobalt,copper, palladium, platinum, and aluminum. The present invention alsocontemplates treatment of layers containing semiconductive materials,such as silicon, polycrystalline silicon, and GaAs. Moreover, the layerstreated can contain mixtures of metallic type materials andsemiconductors.

The preferred structures treated according to the present inventioninclude at least two layers which are adjacent each other and inintimate contact with each other wherein one of the layers is present ontop of the layers. Most preferably the top layer is of a metallic-typematerial.

Particular structures which are especially benefited by the presentinvention includes those which contain an upper layer of a metal whichis very resistant to oxidation, expecially at elevated temperatures,such as gold, which is positioned on top of a layer of a differentmetallic-type material, such as chromium, cobalt or copper, or of asemiconductor, such as silicon, or polycrystalline silicon, or GaAs.When desired, the layers can contain mixtures or alloys of these metals.The process is most advantageous when the structures are of relativelythin layers, such as about 100 to about 5000 A thick, and preferablyabout 500 to about 3000 A thick.

Semiconductor devices employing an upper layer of gold, an intermediatelayer of one or more of the other metals or semiconductors mentionedhereinabove, and a substrate of a semiconductor, such as galliumarsenide are examples of devices which can be treated according to thepresent invention. When fabricating such types of semiconductor devices,metallic electrical contacts are provided on the top surface of the goldat preselected locations and sometimes at the underside of thesemiconductor substrate whereby an electrical connection between the twocan be provided by means of wire. This processing requires the use ofelevated temperatures, such as at least about 100° C., and generally upto about 500° C. Most of the structures are prepared at temperatures ofabout 200° to about 350° C.

However, as discussed hereinabove, when such structures are subjected tothese elevated temperatures, interdiffusion at the interface between thegold, and the underlying layer, such as chromium, tends to occur. Withrespect to a structure which includes gold as the top surface andchromium as the intermediate or underlying layer, once interdiffusiontakes place, the chromium tends to quickly migrate to the top surface ofthe gold, thereby causing problems with respect to reliability of thedevice.

For instance, since chromium is much more susceptible to oxidation thanis gold, this migration could eventually result in formation of areas ofchrome oxide on the top surface which would act as an insulator andhinder electrical contact with the top surface.

Migration through the gold is accelerated, since the gold layer asnormally deposited by vapor deposition is polycrystalline, therebyproviding pathways for the chromium or other material to migrate.Accordingly, the present invention is preferably concerned with treatingthose structures wherein the top surface is of a polycrystalline-typematerial. The gold inherently forms polycrystalline structure whenprepared according to vacuum deposition in the film thicknessesexperienced according to the present invention.

In addition, the interdiffusion can further cause the gold to migratethrough the intermediate layer and through the semiconductive substrate,thereby causing short circuiting.

In order to minimize and, preferably, substantially eliminate thisinterdiffusion at the interface, it has been discovered, according tothe present invention, that when the structure is subjected to elevatedtemperatures (e.g. about 100° C. and above), at least one surface of atleast one of the metallic-type electrical conductors or semiconductors,and preferably at least the top surface of the upper layer, is exposedto a gas, wherein the gas contains at least one type of molecule whichchanges the surface potential of the layer exposed to the gas byincreasing its work function when it is more electronegative than thelayer adjacent it, or which decreases its work function when it is lesselectronegative than the layer adjacent it.

When the gas is a mixture of gases, the relative amount of the gas inthe mixture which changes the surface potential in the manner desired issufficient so that the effect of the mixture of gases is such as tochange the surface potential in the desired manner or direction(i.e.--decrease or increase the work function as required).

The determination of whether a particular gas or mixture of gasesincreases or decreases the work function of a particular surface can bereadily determined by persons skilled in the art by the techniquesreferred to hereinabove. In addition, many gases have already beentested and reported as to the manner in which such alter the workfunction of particular materials. Along these lines, see Somorjai,G,A.--Principles of Surface Chemistry, Prentice-Hall, Inc., EnglewoodCliffs, N.J. (1972), pages 249, 158, and 159, disclosure of which isincorporated herein by reference.

The preferred structures treated according to the present invention arepreferably treated by positioning the structure in a chamber with thesupporting substrate facing downward on a support and the top surface ofthe upper layer facing upwards with the gas being supplied through thechamber. The gas upon contact with the upper surface of apolycrystalline layer, such as gold, will diffuse rapidly down to theinterface, since the grain boundaries of the polycrystalline structuretend to act as pathways.

Normally, the structures are subjected to elevated temperatures at anyone time for a few minutes to about two hours, and more usually fromabout 10 minutes to about 1 hour under normal processing and/oroperating conditions.

It has been found, according to the present invention, that structureswherein the upper layer is gold and the intermediate layer is a materialless electronegative than the gold, such as chromium, silicon, cobalt,and copper, that both carbon monoxide and hydrogen increase the workfunction of the gold layer and are suitable for carrying out the presentinvention. The carbon monoxide is much more effective than the hydrogen.In addition, it is noted that steam, air, and oxygen actually reduce thework function of the surface of the gold and, therefore, result in anincrease in the interdiffusion of the gold and the material adjacentthereto.

It is desirable to use mixtures of carbon monoxide and another gas, suchas air, so long as sufficient carbon monoxide is present in the mixtureto effect the desired minimization of the interdiffusion. For instance,it was noted that a ratio of 1:1 air and carbon monoxide actuallyprevented the interdiffusion of gold and chromium.

In addition, it has been found, according to the present invention, thatstructures of platinum on top of a layer of material which is lesselectronegative than the platinum, such as chromium, can be processedaccording to the present invention employing oxygen or air. Oxygen andair actually increase the work function of platinum as opposed to itseffect on the work function of gold.

In addition, certain preferred aspects of the present invention are alsodiscussed in IBM Technical Disclosure Bulletin, Volume 21, No. 10, March1979--"Passivating Process for the Thin Gold Layer on Silicon", by C. A.Chang, disclosure of which is incorporated herein by reference.

In order to further illustrate the present invention, the followingexamples are presented.

EXAMPLE I

A structure having a substrate of Al₂ O₃ upon which is provided a layerof about 2000 to about 3000 A chromium and a vapor deposited layer ofabout 2000 to about 3000 A of gold on the chromium is provided. Thestructure (the gold facing upward) is subjected to temperatures of about250° C. for one hour in a mixture of about 1:1 volume ratio of air andcarbon monoxide. After this, the structure is tested for interdiffusionby using He⁺ - ion backscattering spectrometry. This procedure involvescounting the number of He⁺ ions reflected back from the sample using He⁺at 2.8 MeV. and the counts times 10⁻³ are plotted against the channelnumber and reproduced in FIG. 1. Also plotted in FIG. 1 is thebackscattering data achieved from the same structure which has not beensubjected to the elevated temperatures. As seen from FIG. 1, the 1:1ratio of air and carbon monoxide resulted in completely stopping theinterdiffusion.

EXAMPLE II

Example I is repeated except that the structure is heated in air for 1hour at 250° C. The results obtained are presented in FIG. 2, and asapparent from FIG. 2, a comparison of that treatment with the referenceshows a significant interdiffusion of the gold and chromium. Note theyield between the 300 and 400 channel numbers. As noted, for thereference within that range, the yield is about 0. On the other hand,significant yields are obtained when the heating is carried out in air.

EXAMPLE III

Example I is repeated except that the structure is heated in a mixtureof about 5 parts by volume of air per 1 part by volume carbon monoxideat 250° C. for about 1 hour. The results obtained are shown in FIG. 2.As apparent from FIG. 2, the air and carbon monoxide mixture of 5:1resulted in some reduction of the interdiffusion of the gold andchromium, but not the complete elimination of such as achieved by aratio of 1:1 of air and carbon monoxide.

What is claimed is:
 1. A method for reducing the interdiffusion when atelevated temperature of a first material and a second material which isadjacent to and in intimate contact with the first material, wherein theelectronegativity of the first material is different from theelectronegativity of the second material, and wherein each of the firstmaterial and the second material is a metallic-type electricalconductive material or a semiconductive material or a mixture thereofwhich method comprises when at elevated temperature of at least about100° C. exposing at least one surface of said first material containingCO or O₂ or both, provided that the gas increases the work function ofthe first material when it is more electronegative than the secondmaterial or decreases the work function of the first material when it isless electronegative than the second material, and thereby reducing theinterdiffusion of the first and second materials at their interface andat elevated temperature.
 2. The method of claim 1 wherein the firstmaterial is located on top of the second material.
 3. The method ofclaim 2 wherein said first material is a layer comprising ametallic-type electrical conductive material.
 4. The method of claim 2wherein the first and second materials are layers about 100 to about5000 A thick.
 5. The method of claim 2 wherein the first and secondmaterials are layers about 500 to about 3000 A thick.
 6. The method ofclaim 3 wherein said metallic-type electrical conductive material ispolycrystalline gold.
 7. The method of claim 6 wherein said secondmaterial is a layer of a material selected from the group of chromium,cobalt, copper, silicon, and mixtures thereof.
 8. The method of claim 1wherein said second material is a layer of a material selected from thegroup of chromium, cobalt, copper, silicon, and mixtures thereof.
 9. Themethod of claim 2 wherein the second material is located on top of asemiconductive substrate.
 10. The method of claim 9 wherein saidsubstrate is gallium arsenide.
 11. The method of claim 1 wherein saidelevated temperature is at least about 100° C.
 12. The method of claim 1wherein said elevated temperature is up to about 500° C.
 13. The methodof claim 1 wherein said elevated temperature is about 200° to about 350°C.
 14. The method of claim 7 wherein said gas is a mixture of carbonmonoxide and air.
 15. The method of claim 6 wherein said gas is amixture of carbon monoxide and air in a ratio of about 1:1.
 16. Themethod of claim 6 wherein said second material is less electronegativethan said gold.
 17. The method of claim 7 wherein said substrate isgallium arsenide.
 18. The method of claim 3 wherein said metallic-typeelectrical conductive material is platinum.
 19. The method of claim 18wherein said second material is less electronegative than said platinum.20. The method of claim 19 wherein the gas contains O₂.